Telecommunications apparatus and methods for handling split radio bearers

ABSTRACT

A method for use in a wireless telecommunications network, the method including: when a secondary base station exhausts a supply of unique parameter sets used in security ciphering of received split radio bearers, the secondary base station sends a notification to a master base station of a requirement to alter data handling resources allocated for handling split radio bearers received from the secondary base station in one or both of the master base station and a terminal device; and the master base station, in response to the notification, operating to effect an alteration in the said data handling resources.

BACKGROUND Field

The present disclosure relates to apparatus and methods for handlingsplit radio bearers in a telecommunications network.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Mobile telecommunications systems, such as those based on the 3GPPdefined UMTS and Long Term Evolution (LTE) and Long Term EvolutionAdvance (LTE-A) architectures, are applicable to communications betweennetworked user devices such as mobile telephones, and more widely alsoto applications such as the Internet of Things. The networked devicesare supported by a telecommunications network comprising base stationsof various configurations offering connection coverage over particularareas, known as cells, and the base stations are in turn supported by acore network. Transmission of data and other signalling between thesevarious entities is achieved by the use of radio bearers which transportthe required messages, for example as a signalling radio bearer whichcarries operational information for the entities, or a data radio bearerwhich carries data. In some instances a bearer is direct between twoentities (a base station and a user device), one sending the message andthe other receiving it. In other cases, a split bearer may be used,allowing a received message to be divided between the radio handlingresources of two receiving entities. Hence, a split bearer is dividedbetween two base stations, each of which passes its part of the splitbearer to a user device. The user device is appropriately configuredwith resources to handle data received from each base station so that itcan manage the split bearer. This arrangement shares resources andenhances speed and efficiency.

The splitting of bearers in this way requires consideration of theoperation of the resources in both of the entities between which thebearer is split, and the resources in the user device corresponding toeach entity. Resources for each side of the split bearer should bemaintained in an operational state for successful handling of themessage.

SUMMARY

The present disclosure can help address or mitigate at least some of theissues discussed above.

Respective aspects and features of the present disclosure are defined inthe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the present technology. The described embodiments,together with further advantages, will be best understood by referenceto the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein likereference numerals designate identical or corresponding parts throughoutthe several views, and wherein:

FIG. 1 shows a schematic representation of an example mobiletelecommunications network or system;

FIG. 1A shows a schematic representation of an example user equipmentfor use in a network such as the FIG. 1 example;

FIG. 1B shows a schematic representation of an example base station foruse in a network such as the FIG. 1 example;

FIG. 2 shows a schematic representation of base stations and userequipment arranged for dual connectivity;

FIG. 3 shows a schematic representation of base stations and userequipment with cells arranged in groups;

FIG. 4 shows a schematic representation of an example user planeprotocol stack for dual connectivity;

FIG. 5 shows a schematic representation of a modified example user planeprotocol stack for dual connectivity;

FIG. 6 shows a ladder diagram of steps in a reconfiguration (“change”)procedure for use following a protocol counter rollover in a secondarycell group within a network;

FIG. 7 shows a ladder diagram of steps in an example procedure for usefollowing a protocol counter rollover in a secondary cell group within anetwork;

FIG. 8 shows a ladder diagram of steps in a further example procedurefor use following a protocol counter rollover in a secondary cell groupwithin a network;

FIG. 9 shows a schematic representation of base stations and userequipment arranged using primary cells within cell groups;

FIG. 10 shows a schematic representation of an example protocol stackfor LTE-NR interworking with a split bearer;

FIG. 10A shows a diagrammatic representation of example options forhandling a split bearer between entity resources;

FIG. 11 shows a ladder diagram of steps in an example procedure for usein the event of a primary secondary cell change; and

FIG. 12 shows a ladder diagram of steps in a further example procedurefor use in the event of a primary secondary cell change.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a schematic diagram illustrating some basic functionalityof a mobile (cellular, wireless) telecommunications network/system, inthis example operating generally in accordance with LTE principles, andwhich may be adapted to implement embodiments of the disclosure asdescribed further below. Various embodiments of FIG. 1 and theirrespective modes of operation are well-known and defined in the relevantstandards administered by the 3GPP® body, and also described in manybooks on the subject, for example Holma and Toskala [1]. It will beappreciated that operational aspects of the telecommunications networkwhich are not specifically described below may be implemented inaccordance with any known techniques, for example according to therelevant standards and known variations thereof. Furthermore, it will beappreciated that whilst some specific examples described herein mayrefer to implementations based around particular 3GPP implementations,the same principles can be applied regardless of the underlyingoperating principles of the network. That is to say, the same principlescan be applied for wireless telecommunications networks operating inaccordance with other standards, whether past, current or yet to bespecified.

The network 100 in FIG. 1 includes a plurality of base stations 101connected to a core network 102. Each base station provides a coveragearea or cell 103 within which data can be communicated to and fromterminal devices or user equipment 104 within the respective coverageareas 103 via a radio downlink DL. Data is transmitted from userequipment 104 to the base stations 101 via a radio uplink UL. The uplinkand downlink communications are made using radio resources that may beused by the operator of the network. The core network 102 routes data toand from each user equipment 104 via the respective base stations 101and provides functions such as authentication, mobility management,charging and so on. Regarding terminology, terminal devices may also bereferred to as mobile stations, user equipment (UE), user terminal,terminal, mobile radio, mobile terminal, mobile device, or simplydevice, and so forth. Base stations may also be referred to transceiverstations, nodeBs, e-nodeBs, eNBs and so forth.

FIG. 1A shows a schematic representation of an example of a userequipment 104. The user equipment 104 comprises a transceiver unit 104Afor transmission and reception of wireless signals and a processor unit104B configured to control the user equipment. The processor unit 104Bmay comprise various sub-units for providing functionality in accordancewith embodiments of the present disclosure as explained further herein.These sub-units may be implemented as discrete hardware elements or asappropriately configured functions of the processor unit. Thus theprocessor unit 104B may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless telecommunications systems. The transceiver unit104A and the processor unit 104B are schematically shown on FIG. 1A asseparate elements for ease of representation. However, it will beappreciated that the functionality of these units can be provided invarious different ways, for example using a single suitably programmedgeneral purpose computer, or suitably configured application-specificintegrated circuit(s)/circuitry. It will be appreciated that the userequipment will in general comprise various other elements associatedwith its operating functionality, for example a power source, userinterface, and so forth, but these are not shown in FIG. 1A in theinterests of simplicity.

FIG. 1B shows a schematic representation of an example of a base station101. In a network such as that in FIG. 1, each base station 101 may befunctionally identical but each serves one or more differentgeographical area (cells 103). In some examples, base stations may beconfigured for operation in different related, or interworking,architectures, in an arrangement known as dual connectivity. In general,though, each base station 101 comprises a transceiver unit 101A fortransmission and reception of communications between the base stationand any user equipment 104 in its cell, and the core network 102. A basestation 101 further comprises a processor unit 101B configured tocontrol the base station 101 to operate in accordance with embodimentsof the present disclosure as described herein. The processor unit 101Bmay again comprise various sub-units for providing functionality inaccordance with embodiments of the present disclosure as explainedherein. Theses sub-units may be implemented as discrete hardwareelements or as appropriately configured functions of the processor unit.Thus, the processor unit 101B may comprise a processor unit which issuitably configured/programmed to provide the desired functionalitydescribed herein using conventional programming/configuration techniquesfor equipment in wireless telecommunications systems. The transceiverunit 101A and processor unit 101B are schematically shown in FIG. 1B asseparate elements for ease of representation. However, it will beappreciated that the functionality of these units can be provided invarious different ways, for example using a single suitably programmedgeneral purpose computer, or suitably configured application-specificintegrated circuits(s)/circuitry. It will appreciated that the basestation 101 will in general comprise various other elements, for examplea power supply, associated with its operating functionality.

In particular, the processor units of user equipment and base stationsinclude resources for handling radio bearers. The resources may includea protocol stack comprising layers including a PDCP (packet dataconvergence protocol), a RLC (radio link control) and a MAC (mediumaccess control), where the layers may be dedicated to particular typesof radio bearer, or may be shared. Under particular events in thenetwork operation, one or more parts of the resources may need to bereset or re-established for continued operation, which herein isreferred to generally as reconfiguration, or alternatively asalteration, indicating some change in in the resources, includingresetting, re-establishment, clearing, removing from use and other likeprocedures that alter the way in which the resources are able to handleone or more radio bearer types. The procedures for resetting andre-establishing are well-understood, and specified in the 3GPPstandards. For example, the MAC reset procedure is specified in section5.9 of 3GPP specification TS 36.321, the RLC re-establishment procedureis specified in section 5.4 of 3GPP specification TS 36.322, and thePDCP re-establishment is specified in section 5.2 of 3GPP specificationTS 36.323. On a high level, a layer is reset during reset orre-establishment, but different terminology is used in the differentspecifications for the various protocol layers.

As is well understood, in wireless telecommunications networks such asan LTE type network, there are different Radio Resource Control (RRC)modes for terminal devices, including governing the connection statebetween the terminal device and a base station. These include an idlemode and a connected mode. Generally speaking, in RRC connected mode aterminal device is connected to a base station in the sense of beingable to receive user plane data from the base station, whereas in RRCidle mode the terminal device is unconnected to a base station in thesense of not being able to receive user plane data from the basestation. However, in idle mode the terminal device may still receivesome communications from base stations, for example, referencesignalling for cell reselection purposes and other broadcast signalling.

While the FIG. 1 example network shows all base stations (eNBs) as beingthe same, and each supporting one cell, in some networks and systemsother arrangements may be used. For example, in Release 12 of the 3GPPstandard governing the LTE architectures, the concept of dualconnectivity (DC) was introduced. In dual connectivity, base stationsare specified as being either a master base station or a secondary basestation, and user equipment can connect with both.

FIG. 2 shows a schematic representation of the control planearchitecture specified for dual connectivity. A master base station 105(designated MeNB) and a secondary base station 106 (designated SeNB)communicate via a control plane using X2-C layer protocol. However,unlike the description above in which any base station is involved inhandling RRC communications, in dual connectivity only the MeNB 105 isdesignated for RRC handling. Consequently, the RRC entity resides in theMeNB 105, and communication with the core network in the form of amobility management entity 107 (MME) via an S1-MME protocol layerterminates in the MeNB 105.

Also, it is possible for a base station, being a master or a secondaryeNB, to support more than one cell. FIG. 3 shows a schematicrepresentation of part of a network having a MeNB 105 supporting threecells 105A, 105B and 105C, and a SeNB 106 supporting three cells 106A,106B and 106C. A user equipment UE 104 has access to cells from botheNBs, indicated as the group 108. Within the group 108, one cell can bea primary cell, from the MeNB cells. Within the group 108 relating tothe UE 104, the MeNB cells 105A and 105B are designated as a master cellgroup MCG, and the SeNB cells 106A and 106B are designated as asecondary cell group SCG. The UE 104 has access to the cells of MCG andthe two cells of the SCG, indicated by the overlapping cell areas inFIG. 3.

A purpose of the dual connectivity arrangement is to enable sharing andcombining of resources belonging to different base stations. Thissharing is expressed in the concept of split bearers.

FIG. 4 show a schematic representation of an example user plane protocolstack for a dual connectivity arrangement. Typically, an incomingmessage arrives via a bearer and is handled by the various protocollayers defined within the LTE architecture. Once master and secondaryeNBs are defined and grouped in dual connectivity, one can furtherdesignate a bearer intended for the MeNB 105 as a master cell groupbearer, MCG bearer 109, and a bearer intended for the SeNB 106 as asecondary cell group bearer, SCG bearer 110. A bearer arrives via the S1protocol layer, is handled by the eNB's resources in turn by a packetdata convergence protocol (PDCP), then a radio link control (RLC)protocol, and then the medium access control (MAC) layer. As shown inFIG. 4, each eNB 105, 106 has these resource layers to handle receivedbearers.

In addition to the MCG bearer and the SCG bearer, dual connectivitydefines a third, split bearer, for the purpose of sharing resources inthe MeNB and the SeNB on the network side of the telecommunicationssystem. A split bearer 111 is delivered to a PDCP in the MeNB 105, andthe MeNB 105, at the PDCP, then controls a split or division of thesplit bearer's data between the MeNB 105 and the SeNB 106. Data for theMeNB 105 is passed to the MeNB's RLC and then its MAC, and data for theSeNB 106 is passed from the MeNB 105, using the X2 protocol layer, to anRLC in the SeNB and then to the MAC of the SeNB.

In order to be able to handle a message carried by a split bearer onceit is passed on from the two eNBs, a UE is provided with two MACentities, a master cell group MAC (MCG MAC) and a secondary cell groupMAC (SCG MAC), plus corresponding RLC and PDCP. These are included inthe resources of the UE for split bearer handling.

As mentioned above, only the MeNB has a RRC entity, so signalling radiobearers for RRC are transported over the MCG only, i.e. by MCG bearer.The SCG is not involved in the transporting of RRC messages. For UEsconfigured for dual connectivity and split bearer transport, usertraffic from the core network can be received at the MeNB as a splitbearer, and then divided between the MeNB and the SeNB for handling andpassing to the UE. Any traffic on a SCG bearer is received from the corenetwork at the SeNB and transported using resources of the SeNB to theUE.

In the context of LTE, further details regarding dual connectivity canbe found in the 36.300 specification at sections 6.5 and 7.6, and alsoin 3GPP TR 36.842.

As can be seen from FIG. 4, a bearer arrives at the PDCP protocol layer.The PDCP is involved in security of the data traffic, includingciphering using a key. Each PDCP in each network entity (eNBs and UEs,for example) will use its own key; these are regularly updated. The MeNBmay use a key designated as KeNB, while the SeNB may use a keydesignated as SKeNB. Other parameters are utilised by the PDCP togetherwith the key to effect security; these include a numerical counter togenerate successive numbers in a sequence of count values. Hence thereis a set of parameters, used in a security algorithm to perform theciphering. Each set of parameters, one for each successive number fromthe count value, is used only once for ciphering, to maintain security.The count value has a maximum number that can be generated, so for agiven key, once this number is reached, there are no new parameter setsavailable for ciphering. Re-use of parameters is undesirable, so it ispreferred to acquire a new key for the PDCP and start the count valuesequence again at its beginning (at zero, for example), to work throughall successive values in the count value sequence with the new key.

The expiration of the numbers available from the counter can be referredto as “rollover”, and hereinafter the disclosure may mention “PDCProllover”, “PDCP counter rollover”, “PDCP count rollover or “countrollover”. The process following rollover, including acquisition of anew key, has a high processing overhead associated with it, and requiresa resetting of the MAC layer for handling of ciphering with the new key.

An example of a possible network configuration for futuretelecommunications is an arrangement comprising an LTE architectureproviding wide (macro) coverage in conjunction with a so-called newradio (NR), referring to current and future telecommunications methodsallowing increased data throughput, such as 4th and 5th generations (4Gand 5G) and further. The type of radio access technology (RAT) used inthe LTE network and the new radio network may be different, but an LTEnetwork and a NR network could interwork, where a benefit of havingconnectivity to both LTE and NR is reduced signalling towards the corenetwork from mobility towards the core network being anchored at the LTEmacro entity, combined with higher throughput made possible be utilisingresources in both LTE and NR. A UE will be configured to operate underboth RATs. In this context, dual connectivity is relevant, such thatMeNBs may be designated from LTE and SeNBs from NR, or vice versa.

Split bearers are therefore also relevant, and a new split bearerconfiguration is considered, namely a secondary cell group split bearer,or SCG split bearer.

FIG. 5 shows a schematic representation of an example user planeprotocol stack utilising a SCG split bearer. As in FIG. 4, a master node105 (in this example in the LTE side) and a secondary node 106 eachreceive their designated bearers, MCG bearer 109 and SCG bearer 110respectively, and these are handled by a PCDP, a RLC and a MAC layer, asbefore. No conventional split bearer is included, however; instead thereis a SCG split bearer 112 which is delivered to the SeNB 106 (labelledSecondary gNB in FIG. 5 to indicate a difference from the eNB of FIG. 4owing to the addition of the NR network). A PDCP in the SeNB 106receives the SCG split bearer 112 and divides the data. Some is retainedin the SeNB, being passed to the RLC and MAC layers. Other data ispassed from the SeNB 106 to the MeNB 105 via an X protocol layer(labelled Xnew to indicate possible change from the X layers within LTE,such as the X1 layer in FIG. 4), and the MeNB 105 handles it with itsown RLC and MAC resources.

The SCG split bearer is proposed in the context of the higher data ratesthat can be handled in an NR architecture. Note this is merely anexample, however, and secondary cell group split bearers are relevant inother contexts also.

FIG. 5 shows SCG split and SCG bearers together, and they may besimultaneously used or supported. SCG bearer can be considered as aspecial case of SCG split bearer, in which 100% of the data traffic isover the SCG and 0% over the MCG. Either or both of the SCG bearer andthe SCG split bearer may coexist alongside the MCG bearer. However,coexistence of the MCG split bearer (as in the FIG. 4 example) and theSCG split bearer is unlikely owing to different transport requirementsand hence a need for a high bandwidth in the user plane anchor. However,any coexistence of bearer types is not relevant to the presentdisclosure, and embodiments and examples addressing the SCG split bearercan be implemented regardless of other secondary node bearers.

However, the coexistence of the MCG bearer and the SCG split bearerimplies that there will be at least one PDCP entity in the MCG, tohandle the MCG bearer, and at least one PDCP entity in the SCG, tohandle the SCG split bearer. Consequently, ciphering will be carried outusing two keys, the KeNB in the MCG PDCP and the SKeNB in the SCG PDCP.The MeNB MAC receives data ciphered with both keys.

Recall the above discussion that mentioned PDCP count rollover. Under3GPP Release 12 for dual connectivity, a “SCG change” procedure isdefined for situations including a rollover of the SeNB PDCP receivingthe SCG bearer. This is defined in section 10.1.2.8.6 of the 36.300standard, and includes the requirement that “During SCG change, MACconfigured for SCG is reset and RLC configured for SCG is re-establishedregardless of the bearer type(s) established on SCG. For SCG bearer,PDCP configured for SCG is re-established. In case of reconfigurationfrom split to MCG bearer, RLC configured for SCG is released. During SCGchange, S-KeNB key is refreshed.”

The SCG change procedure is applicable in a range of scenarios. FIG. 6shows a depiction of the procedure, from standard 36.300 section10.1.2.8.2.

This scope of SCG change procedure is restricted to cells under controlof the SeNB, and therefore in the SCG. However, a SCG split bearer willuse a RLC instance in the MCG (see FIG. 5), and share the MCG MAC withother MCG bearers including SRBs (signalling radio bearers, for RRCsignalling). Also, note that in any change between MCG bearer and(conventional) split bearer there will be no need to reset any of theMCG resources because ciphering for both bearers is done in MCG PDCP(see FIG. 4), and MCG RLC and MCG MAC can continue without any reset.This is not possible for the SCG split bearer, however, because the SCGPDCP in the SeNB will cipher the SCG split bearer, before passing it toresources in the MCG (RLC and MAC in the MeNB).

Hence, a difficulty can arise for SCG split bearers when a PDCP countrollover occurs in the SeNB. Recall from above that a count rolloverinitiates the SCG change procedure, which includes refreshing of theSKeNB key. Resources in the MeNB may then be unable to handle theirallocated part of the SCG split bearer.

Standard 36.300, section 14.1 specifies PDCP count in dual connectivityas: SCG bearers in DC share a common pool of radio bearer identities(DRB IDs) together with the MCG bearers and when no new DRB ID can beallocated for an SCG bearer without guaranteeing COUNT reuse avoidance,the MeNB shall derive a new S-K_(eNB). SeNB indicates to MeNB whenuplink or downlink PDCP COUNTs are about to wrap around and MeNB shallupdate the S-K_(eNB). To update the S-K_(eNB), the MeNB increases theSCG Counter and uses it to derive a new S-K_(eNB) from the currentlyactive KeNB in the MeNB. The MeNB sends the newly derived S-K_(eNB) tothe SeNB. The newly derived S-K_(eNB) is then used by the SeNB incomputing a new encryption key K_(UPenc) which is used with all DRBs inthe SeNB for this UE. Furthermore, when the SCG Counter approaches itsmaximum value, the MeNB refreshes the currently active KeNB, before anyfurther S-K_(eNB) is derived.

From this we can appreciate that in the event of PDCP rollover for a SCGsplit bearer, it is required that the SCG change procedure should beinitiated for the resources under the

SeNB, i.e. SCG RLC and PDCP should be re-established and the SCG MAC isreset. In the scenario of LTR-NR interworking described above, it islikely that PDCP count rollover will happen in the NR PDCP (in otherwords, the SeNB, as in FIG. 5), because the majority of the data trafficwill be pushed using NR (rather than LTE) to take advantage of thehigher throughput. Accordingly, following a similar logic, and becauseSCG split bearer data packets will be ciphered by the SCG PDCP, rolloverof the SCG PDCP suggest a requirement for a change procedure in whichthe MCG RLC should be re-established, and the MCG MAC should also bereset.

Consequently, a proposal to address the issue of PDCP rollover in theSeNB when SCG split bearers are used is to ensure that appropriatehandling of the MCG resources is undertaken, which is some examplesincludes resetting/re-establishing of the MCG RLC and the MCG MAC.

FIG. 7 shows a ladder diagram indicating steps in a first proposal of amethod to achieve this. A network and its entities are configured forSCG split bearer use. For example, the entities comprise a UE 104, aMeNB 105 in a LTE architecture and a SeNB 106 in a NR architecture. In afirst step S1, PDCP count rollover is recognised in the SeNB 106. Instep S2, the SeNB 106 indicates to the MeNB 105 that PDCP rollover hasoccurred so that SeNB resource modification (reconfiguration) isrequired. In response, the MeNB 105 creates a new security key for theSeNB 106, and sends it to the SeNB 106 in step S3. Note that steps S1,S2 and S3 are the same as in the known SCG change procedure. Under theproposal, however, a next step S4 requires the SeNB 106 to additionallyindicate to the MeNB 105 that handling (modification, reconfiguration)of the MCG resources relevant to the SCG split bearer is required, sothat the MCG RLC and the MCG MAC are to be reset; the MeNB 105 performsthis. Finally, in Step S5, the MeNB 105 carries out RRC reconfigurationof the UE 104. This also occurs in the SCG change procedure, in that theUE's SCG MAC is reset and its SCG RLC is re-established, butadditionally here the UE's MCG MAC is reset and its MCG RLC isre-established (recall that the UE is provided with resources for boththe MCG and the SCG for operation under dual connectivity, so it has twoMAC entities, for example). The UE's SCG PDCP for the split SCG beareris also re-established.

However, this solution presents issues in that resetting the MCG MACwill impact the SRBs and DRBs (signalling and data radio bearers)arriving at the MCG. There will be no issue regarding the RLC in theMeNB as there is a single instance of RLC per RB, but the MAC layer isconfigured for a whole cell group (MCG or SCG). Consequently, it wouldbe preferable to avoid resetting the MAC. It is equivalent to moving theUE to RRC idle state, which is clearly problematic.

On the other hand, if the MCG MAC is not reset, one consequence is thatthere may be data packets (herein also “packets”) in the MCG MAC HARQbuffer for the SCG split bearer which will halt a HARQ process or makeit unusable. HARQ, or hybrid automatic repeat request, combineshigh-rate forward error correcting coding and ARQ error control, and isa process undertaken in the MAC, taking packets stored in the HARQbuffer. Resetting the MAC clears the buffer, and hence addresses anyhalt or unusability of a HARQ process. As a consequence, it is importantto consider carefully the prospect of resetting or not resetting the MCGMAC to address an SCG PDCP count rollover.

A second proposal, alternative to that in FIG. 7 is to not reset the MCGMAC, and limit the handling of the MCG resources to a re-establishmentof the MCG's RLC for SCG split bearer only. DRB release for conventionalbearers does not involve a MAC reset, so omitting this procedure isfeasible. Note however that it is assumed that there will be no packetsqueued in the HARQ for a bearer about to be released so that allprocesses continue as usual for the remaining DRBs.

A benefit of not resetting the MCG MAC for a SCG split bearer is thatthere will be no interruption of traffic on the MCG side of the link.However, there may be packets in the MCG MAC related to the SCG splitbearer. An option to manage these is to continue with thetransmission/reception until the HARQ processes have cleared, forexample by setting a timer for the HARQ operation so that it is assumedto have cleared when the timer expires. Then, the MCG MAC discards anyremaining packets related to the SCG split bearer after the MCG RLC hasbeen re-established and the timer has expired (or the HARQ queue isotherwise empty or deemed empty).

For downlink traffic, one may arrange that the SCG PDCP stops sendingpackets to the MCG RLC well in advance of the MCG RLC re-establishment,so that the HARQ queue is empty before the RLC re-establishment isperformed, or will be insufficiently full so as to certainly ornear-certainly be able to empty during operation of the timer. Thenthere is no need to reset the MCG MAC. Under this regime, the UE may benotified well in advance that there is no requirement for its MCG MAC tobe reset. For example, this information may be transmitted in aRRC/MAC/PHY layer message.

For the uplink, the network may reconfigure the UE with a split ratiofor the SCG split bearer of zero percent for the MCG side of itsresources (so that all split bearer traffic is diverted to the SCG side,thus avoiding the MCG side until regular operation after the countrollover has resumed). Alternatively, and similarly, the UE may beconfigured so that no uplink is granted on the MCG, again avoiding theMCG MAC during the critical time around count rollover.

FIG. 8 shows a ladder sequence of messages in an example procedure toimplement the second proposal. The network is as in the FIG. 7 example,with a UE 104, a MeNB 105 and a SeNB 106 configured for SCG split beareroperation. Steps S1 and S2 are as in FIG. 8, but in this example, afterthe SeNB 106 notifies the MeNB 105 that modification is needed followinga PDCP count rollover in step S2, in step S3 a the SeNB 106 stopstraffic to the MeNB 105, and also the MeNB 105 stops the UE's uplink onMCG resources (by no uplink grant or by an explicit indication, forexample). Then, in step S3 b, there is a status report to the SeNBregarding the data in the MAC HARQ buffer, for example that the timerdescribed above has expired or that the buffer is empty (no packets inthe MCG side MAC). Then the procedure moves to step S3 which is the sameas before, namely the provision of a new security key to the SeNB. StepS4 is modified compared to FIG. 7, in that the SeNB 106 indicates areset of the MCG RLC to the MeNB 105 but not an reset of the MCG MAC(this may be a definite instruction not to reset the MAC, or the absenceof an instruction to do a reset). Finally, step S5 is similarlymodified, in that the MeNB 105 performs an RRC reconfiguration of the UE104, which includes re-establishment of the UE's MCG RLC, but does notinclude resetting of the UE's MCG MAC.

So, the SeNB may stop downlink traffic towards the MCG, in step S3 a,and rely on flow control feedback to ensure there are no packets left inthe MAC buffer, as in step S3 b.

Regarding the uplink from the UE, in order to ensure there are no uplinkpackets in the MCG MAC of the UE which is designated for SCG splitbearer, one may implement the following:

-   -   1. The network (that is, the MeNB or the SeNB) explicitly        indicates that it will not provide grant for UL being scheduled        on the MCG MAC of a SCG split bearer. For example, in dual        connectivity operation, an eNB may configure a particular        information element, IE (parameter in a signalling message such        as RRC) as false to indicate that no traffic is sent via SCG.        Currently, there is only an IE defined for this purpose for SCG        traffic. It is therefore proposed to define a new IE that serves        the same function for MCG, that is, a IE which when configured        as false indicates that no traffic is to be sent via MCG.        Alternatively, one can arrange the SCG split bearer so that the        data split threshold can be set at 0 indicating no split so that        all traffic is via the MCG or set at 100 indicating no split in        the other direction so that all traffic is via the SCG. Use of a        threshold of 100 would therefore remove traffic from the MCG as        required. This procedure requires an additional RRC message        being sent before the new key is provided, as in step S3 a in        FIG. 8.    -   2. In order to ensure there are no more packets left and all        buffers are clear, the network may rely on:    -   a. A buffer status report indicating the logical channel group        (LCG) of the split bearer in the MCG. A UE is able to send a        buffer status report indicating a buffer fill level for all        channels in the LCG which is a group of uplink channels. This        could be the status report in step S3 b.    -   b. An RLC (radio link control) status report being enhanced to        indicate that no RLC service data unit is pending for        transmission.    -   c. Expiry of a timer, as discussed above.

Each of these options can be used as a reliable indicator from the UE tothe SeNB that buffers in the UE MCG MAC are clear. One or other can beused in conjunction with stopping the uplink traffic as in paragraph 1above.

-   -   3. Finally, after the new key is delivered to the SeNB in step        S3, the UE is informed about the handling of resources, as        described above with reference to FIG. 8.

A third proposal of a procedure to manage the SeNB PDCP rollover is thatwhen the count rollover happens, the SeNB notifies this fact to theMeNB, and the MeNB moves the UE into a idle state, that is, the UE ismoved to RRC idle. Alternatively, the SCG split bearer is released. Ineither case, traffic is no longer directed to the UE using the SCG splitbearer, so inconsistencies between the SCG MAC and the MCG MAC are notliable to cause any problems.

In the event that in a given new radio (NR) architecture there is no RRCidle state, a fourth proposal is that the UE may instead be moved to aconnected inactive state. This will have the same effect as a move to anidle state.

In a fifth proposal, in response to count rollover occurring or beingabout to occur in the SCG PDCP, the MeNB can perform an intra cellhandover of the UE. The same RRC reconfiguration message might be usedto effect the handover and to establish SCG split bearer. Mobilitycontrol information used for handover already includes resetting of MCGand SCG MAC, and associated RLC and PDCP handling and SCG configuration.

A sixth proposal is for the MeNB or the SeNB to change bearer type fromSCG split bearer to SCG bearer before performing the SCG change on PDCPcount rollover. Following the dual connectivity model, changing thebearer type also involves resetting of MCG MAC.

A seventh proposal is to not reset any resources, on either the MCG orthe SCG. Instead, via RRC the UE is informed about the new key obtainedfor the SCG PDCP. Then the UE uses two keys (the old key and the newkey) for some time before discarding the old key for decryption ofreceived packets (so, when operating in downlink) when it is apparentthat nothing is ciphered with the old key any longer. Other securityinput parameters could also be reset for the new key. The same behaviourmight be employed in a new radio base station for uplink.

This proposal can be enhanced by use of a sequence number from the PDCPcount which is designated as an activation sequence number. The networkestimates a future sequence number that will be at a suitable time forthe keys to be changed, and communicates it to the UE. Then, when thisnumber is reached in the ciphering, both the network and the UE cansynchronise the changeover from the old key to the new key.Alternatively, an activation sequence number may be designated from thesequence of numbers from a count in the RLC layer. The PDCP sequencenumber may become unreliable as it approaches rollover, so use of theRLC count avoids such problems; it is unlikely that both count valueswill rollover at the same time.

As an eighth proposal, one can add an additional parameter as an inputto the security algorithm implemented by the SeNB SCG split bearer toperform ciphering. The additional parameter is added in when the countrolls over, as a way of effectively extending the count by allowingrepeated sequence numbers to be recycled as “new” numbers by theinclusion of the additional parameter. A different additional parametermight be added at each count rollover, or the additional parameter mightbe replaced by a different additional parameter at each count rollover.Following this option, no resource on the MCG side or the SCG side needsto be reset.

An example new parameter is a new counter which is taken in accountwhile generating encryption and integrity keys. Currently, integrityprotection is enabled on SRBs, and it is unlikely that a SRB count wouldroll over. RRC message PDUs (protocol data unit, the output of aprotocol layer to another) are not as frequent as data transmission PDUsin PDCP.

Another example is to add a new bit to the packet header when rolloveroccurs, thus making the combination unique and allowing all existingnumbers from the counter to be used again, each with the new bit.

A further example is to send in the packet header a bitmap related toboth the old key or keys and the new key or keys.

Some of the alternative proposals presented above rely on variousassumptions about the network and its operation and architecture. Forexample, it is assumed that the S1-MME protocol layer (see FIG. 2) willstill be terminated in the MeNB for LTE-NR interworking. Also, thatsecurity keys for SeNB will be derived from KeNB in the MeNB usingexisting dual connectivity procedures (see supply of the new key to theSeNB in FIGS. 7 and 8). However, security keys may be provided to the NRPDCP directly from S1-MME for the SCG split bearer. If so, the corenetwork would need to be involved in provision of the new key.

It has been assumed that the procedures described herein are triggeredby the SeNB. However, the various proposal and techniques for resourcehandling are also applicable in the case of initiation by the MeNB.

We have assumed also that SCG resources related to the SCG split bearerwill be reset or re-established. However, if the MCG side can survivewithout reset then the SCG side of resources can be saved from resetalso.

In an LTE-NR interworking scenario, the various proposals are equallyvalid if the NR is the master instead of the LTE, except if the RLCentity is not agreed as part of the NR protocol stack, in which case theMCG RLC becomes irrelevant.

The proposals consider a single RRC entity in the LTE MeNB. However,each proposal is applicable regardless of the number of RRC entities orstate machines, or the way RRC messages may be transported over NR(using L2 protocol stacks from MCG or SCG).

While the invention has been presented in terms of PDCP count rollover,the proposals may also be applied in situations where the rollover orexpiry of any parameter will risk or prevent the uniqueness of securityalgorithm input parameters being maintained. For example, ciphering mayrely on a key which is time-sensitive, so that expiry of the key is thetrigger for the various procedures.

Note that SCG may operate in licensed or unlicensed bands.

Thus far, we have considered issues arising from PDCP rollover and thesubsequent key change from the SCG change procedure in the context ofSCG split bearers used in a dual connectivity arrangement such as LTE-NRinterworking. Within the same framework, we can also consider a furtherchange procedure which can have similar implications regarding resourcereconfiguration. This is a PSCell change procedure.

Recall the network comprising a master cell group MCG and a secondarycell group SCG, respectively under control of a master eNB, MeNB, and asecondary eNB, SeNB. Within each group, we can designate a primary cell.So, the MCG has a primary cell PCell, and the SCG has a primary cell,the PSCell for primary secondary cell. The PSCell handles or controlsuplink signalling within the cells of the SCG and the PCell handles orcontrols uplink signalling within the cells of the MCG. Any additionalcells in the MCG are called secondary cells, Scells, while anyadditional cells in the SCG are called secondary secondary cells,SSCells.

FIG. 9 shows a schematic representation of such a network. The corenetwork MME 207 supports a MeNB 205 which provides a PCell 205A. In thisexample the LTE provides the master. New radio NR provides thesecondary, so that a secondary SeNB provides a PSCell 206A. SCells andSS Cells are not shown for clarity. A user equipment 204 communicateswith both the MeNB 205 and the SeNB 206. A SCG split bearer 112 is usedto deliver data to the SeNB, where the split bearer 112 is dividedbetween the resources of the SeNB and the MeNB before being transportedby each base station to the user equipment 204, as before.

The PSCell is able to be changed, so that a different cell in the SCGbecomes the primary, i.e. a SSCell becomes the new PSCell and theoriginal PSCell becomes a SSCell. In dual connectivity, this change alsotriggers the SCG change procedure discussed above (and depicted in FIG.6). Cells in a NR SCG have a small size, owing to operation at higherradio frequencies for NR compared to LTE, so mobility of entities willlead to frequent PSCell changes. Note that PCell change is alsopossible.

Various factors have relevance when considering how to manage a PSCellchange. In this scenario, the physical uplink control channel PUCCH onthe SCG is configured on the PSCell. Physical channel reconfiguration isrequired when a new cell takes the role of a PSCell, so that PUCCHresources are configured on the new PSCell, if previously notconfigured. PUCCH resources are not configured for all SCells in LTE. NRmay follow the same principle, or may allow a PUCCH-like control channelon all configured SCells. Both the possibilities are within the currentscope.

In dual connectivity, there is no linkage between the SKeNB key and theSCG counter, and the PSCell identity (ID). So, PSCell change isindependent of SCG security. Conversely, the PCell Cell ID is used forKeNB calculation and NAS (non-access stratum) information is taken fromthe PCell (standard 36.300 section 7.5). Hence, PSCell change and PCellchange procedures can differ from security point of view; there is norequirement for the procedures to be the same.

During SCG change, the SKeNB key is refreshed as described above. ForNR, it is possible that handover and security procedures may beseparated, so in the future, PSCell change (for handover or caused bymobility) may be performed without a SKeNB change.

The random access (RA) procedure, by which a UE accesses the network, isrun on the PSCell. RA procedure requires new time alignment, so therehas been a requirement to reset resources during the dual connectivityPSCell change procedure. However, a RACH (random access channel)-lesshandover procedure where source and target cells are synchronized hasrecently been proposed, so that in the future the RA procedure may notalways be required, or if required have no associated resource reset

Radio Link Monitoring is performed on the PSCell. A change in RLMconfiguration is required at PSCell change.

Uplink thresholds for traffic separation between the MCG and the SCG areconfigured in PDCP and these thresholds are provided by the LTE eNB.

In the context of NR, one may assume, in some cases, a radio accessnetwork (RAN) in which multiple data or distribution units (DU) areconnected to a single control unit (CU) to provide the secondary cellgroup. Change of the PSCell is effected by allocating a different DU toprovide the PSCell. Therefore, one can consider how to perform a PSCellchange for non-standalone for a SCG split bearer, with multiple DUsconnected to a CU on the NR, secondary, side.

PSCell change in dual connectivity is handled by the SCG changeprocedure, described above.

FIG. 10 shows a schematic representation of the protocol instances forLTE-NR tight internetworking in an example case where a UE is connectedto a LTE macro and to a SeNB DU. LTE is the master and NR is SeNB. Inthis example, protocol stacks for two bearers are shown, the MCG bearer109 and SCG split bearer 112. The MCG bearer 109 has associated PDCP andRLC entities. The SCG split bearer 112 has resources in the CU of SCGPDCP, SCG RLC and SCG MAC. Owing to the split, it also has RLC resourcesin the MCG side. The MCG MAC is common for both the bearers, and handlesthe MCG bearer and the part of the SCG split bearer which is passed tothe MCG by the SCG PDCP, as before.

In the earlier scenario, the SCG change procedure was triggered by theSCG PDCP counter rolling over, requiring a new key for the SCC PDCP andhence incompatibility between resources for the SCG split bearer on theSCG side and resources for the other part of the SCG split bearer on theMCG side.

In the current scenario, the SCG change procedure is triggered by aPSCell change. This gives a new SCG PDCP (with a different key and otherciphering algorithm parameters) on the SCG side, again givingincompatibilities between resources for the SCG split bearer on the SCGand the MCG side.

Owing to similarities in the two situations, one can apply various ofthe proposals outlined above for PDCP count rollover to the PSCellchange situation. In particular, each of the first, second, third,fourth and fifth proposals is readily applicable to dealing with PSCellchange. Hence, rather than the SCG split bearer PDCP count rollover inthe SeNB being the trigger for the various methods of resource handlingand management, the PSCell change is the trigger (arising, for example,from entity mobility requiring a different cell in the SCG beingdesignated as the PSCell).

Other methods are also proposed to address the PSCell change situation.A dual connectivity SCG change procedure results in PDCPre-establishment, with the associated implication that the MCG RLC andMCG MAC need to be re-established and reset, leading to SRB disruption(as discussed above). However, considering LTE-WLAN aggregation wherebysecurity and mobility procedures are separated, and a future C-RANarchitecture of NR such as in FIG. 10, PDCP re-establishment may not benecessary for PSCell change.

The FIG. 10 example is one possibility for a future architecture ofcentralised deployment, in which the SCG split bearer is divided afterhandling by the receiving PDCP. Other options are outlined in the 3GPPstandard specification TR 38.801 covering non-centralised deployment(like the LTE architecture) and co-sited deployment (with LTE). FIG. 10ashows a diagrammatic representation of possible options for thesplitting of bearers to be shared between resources. For both uplink anddownlink, the split (for RRC and data) could be arranged at any of thesuccessive protocol layers, so that bearer handling can be sharedbetween two sets of resources A and B. Hence, various options 1 to 8 arecontemplated, respectively for splitting before PDCP (or after for theopposite link direction), between PDCP and high level RLC, between highlevel RLC and low level RLC, between low level RLC and high level MAC,between high level MAC and low level MAC, between low level MAC and highlevel PHY (physical layer), between high level PHY and low level PHY, orbetween low level PHY and RF.

Hence one can consider further the alternatives of both PDCPre-establishment and PDCP maintenance when addressing the issues ofresource handling for PSCell change.

Firstly, consider that the PDCP in the SCG is to be maintained.

A proposed approach is that, assuming PDCP is not re-established forPSCell change within a CU, the PDCP stops sending the traffic to the RLCprotocol layers in both the SCG and the MCG. Information exchangebetween the UE and the network may be necessary in order to ensure thatboth sides are time-synchronised.

FIG. 11 shows a ladder diagram of steps (message sequence) in an exampleof such a method. The network includes a UE, a MeNB in LTE, and CU andDU SeNBs in NR. Firstly, the RRC entity(ies) in the MeNB or SeNB decidebased on measurements that a PSCell change is required. UE measurementsare reported to either a) MeNB or b) SeNB directly or c) received byMeNB and forwarded to SeNB over the X2New interface. Particular valuesof or changes in the measurements, such as those caused by UE mobility,can indicate a requirement for a new PSCell. Then the RRC informs thePDCP and RLC/MAC about the PSCell change decision, and may ask to stopthe traffic. If SeNB is the entity which makes a decision about PSCellchange, then SeNB RRC informs the MeNB (MCG-RLC, MCG MAC) over X2 andthe UE over RRC. The UE RRC internally informs PDCP and RLC/MAC entitiesabout PSCell change. Next, the SCG-PDCP entity in the UE (for theuplink) and in the SeNB (for the downlink) stops traffic towards boththe MCG and SCG. This is for the purpose of obtaining an empty HARQbuffer in the MAC, and can be implemented as discussed previously underthe second proposal for the first scenario, by configuring uplinkthresholds and informing MeNB for downlink traffic, and receivingfeedback from both the MeNB and the UE when buffers are empty. Then, theSCG RLC entities in the UE and the SeNB are re-established and the SCGMAC is reset. The PDCP is informed once it is complete. Finally, thePDCP restarts the traffic and may inform RRC about the completion of theprocedure. If reordering function is in the PDCP only then NR-RLC(SCG-RLC in this example) may also not need a reset.

In an alternative in which the PDCP is re-established, PSCell change ishandled locally within the SeNB but an explicit indication is necessaryfor the UE and MeNB such that it does not result in resetting of MCGMAC. This proposal assumes that PDCP is re-established. FIG. 12 shows aladder diagram of steps (message sequence) in an example of such amethod. Comparison with FIG. 11 shows that the methods are the sameexcept for re-establishment of the SCG PDCP. A different RRCreconfiguration message for the UE is needed, to instruct re-establishof the SCG PDCP in the UE. Stopping of traffic is still required sincethe MCG-MAC is not to be reset.

As a final proposal, we consider a situation in which the SCG MAC in theNR side is not reset. One difference compared to above is that the SCGMAC has always been assumed to be reset because the assumption taken isthat one DU controls a single cell, and a single MAC entity exists perDU. However, multiple cells may share a single MAC entity (scheduler),and UE NR SCG-MAC is able to support multiple cells. If both source andtarget cell are controlled by the same MAC-DU then cell change may takeplace via MAC level signalling. Alternatively, HARQ processes areseparated per bearer in NR MAC, so resetting the resources specific toone bearer does not impact others.

This implies no traffic stopping procedure between the UE, MeNB andSeNB, a difference from the FIGS. 11 and 12 examples. Also, there is noneed for RRC/X2 signalling related to a traffic stop/start, and resetare expected. The SeNB changes the cell internally, taking measurementsor any other internal criteria (e.g. load balancing on uplink/downlinkcontrol channels) into account, and uses MAC control element or physicallayer signalling or similar means (e.g. RLC or PDCP control PDU) tonotify the UE. The change of cells is expected to be quick enough thatno interruption is noticeable on the MCG side and all entities are keptwithout reset or interruption. This is an improvement compared to theschemes of FIGS. 11 and 12.

These examples may also be applied for mobility within a NR MCG as well.

We have assumed in these examples that LTE is the master and NR is thesecondary. For a deployment where NR is master and LTE is secondary, itis assumed that NR CU-DU split may happen and LTE may or may not supportC-RAN architecture. No difference is foreseen between the two cases.

There has been described a method for use in a wirelesstelecommunications network, the mobile telecommunications networkcomprising: a core network; base stations supported by the core networkand each providing wireless connectivity within at least one basestation cell where the cells are arranged into a master cell group undercontrol of a master base station and a secondary cell group undercontrol of a secondary base station; and a terminal device configured tocommunicate wirelessly with the base stations including by the use of asplit radio bearer receivable at the secondary base station forsplitting between the secondary base station and the master base stationbefore delivery to the terminal device; the method comprising: when thesecondary base station exhausts a supply of unique parameter sets usedin security ciphering of received split radio bearers, the secondarybase station sends a notification to the master base station of arequirement to alter data handling resources allocated for handlingsplit radio bearers received from the secondary base station in one orboth of the master base station and the terminal device; and the masterbase station, in response to the notification, operating to effect analteration in the said data handling resources.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, define, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

Respective features of the present disclosure are defined by thefollowing numbered paragraphs:

Paragraph 1. A method for use in a wireless telecommunications network,the mobile telecommunications network comprising: a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and a terminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device; themethod comprising: when the secondary base station exhausts a supply ofunique parameter sets used in security ciphering of received split radiobearers, the secondary base station sends a notification to the masterbase station of a requirement to alter data handling resources allocatedfor handling split radio bearers received from the secondary basestation in one or both of the master base station and the terminaldevice; and the master base station, in response to the notification,operates to effect an alteration in the said data handling resources.Paragraph 2. The method of paragraph 1, in which the parameter sets aremade unique by inclusion of successive numbers from a counter in apacket data convergence protocol that receives, ciphers and splits thesplit radio bearer, and exhaustion of the supply of unique parametersets occurs when the counter rolls over from its final number back toits starting number.Paragraph 3. The method of paragraph 1 or paragraph 2, in which: themaster base station effects the alteration in said data handlingresources by re-establishing its radio link control protocol layer andresetting its medium access control protocol layer, and sending areconfiguration message to the terminal device to effectre-establishment of its radio link control protocol layer and resettingof its medium access control protocol layer.Paragraph 4. The method of paragraph 1 or paragraph 2, in which: themaster base station effects the alteration in said data handlingresources by re-establishing its radio link control protocol layer andnot resetting its medium access control protocol layer, sending areconfiguration message to the terminal device to effectre-establishment of its radio link protocol layer but not resetting ofits medium access control layer.Paragraph 5. The method of paragraph 4, further comprising: arrangingthat a hybrid automatic repeat request buffer in the medium accesscontrol protocol of the said data handling resources in the master basestation is empty before re-establishing its radio link control layer.Paragraph 6. The method of paragraph 5, in which arranging for thebuffer to be empty includes stopping downlink data traffic to the masterbase station for a predetermined time period following exhaustion of thesupply of unique parameter sets.Paragraph 7. The method of paragraph 5, in which arranging for thebuffer to be empty includes stopping uplink data traffic to the masterbase station for a predetermined time period following exhaustion of thesupply of unique parameter sets.Paragraph 8. The method of any one of paragraphs 5 to 7, furthercomprising sending a status report to the secondary base station toindicate an empty status of the buffer, before the secondary basestation sends the notification to the master base station.Paragraph 9. The method of paragraph 1 or paragraph 2, in which themaster base station effects the alteration in said data handlingresources by placing the terminal device into a state in which its saiddata handling resources can no longer handle the split radio bearers.Paragraph 10. The method of paragraph 9, in which placing the terminaldevice into a state comprises moving the terminal device to an idleradio communication mode.Paragraph 11. The method of paragraph 9, in which placing the terminaldevice into a state comprises releasing the terminal device fromconnection with the split radio bearers.Paragraph 12. The method of paragraph 9, in which placing the terminaldevice into a state comprises moving the terminal device to an inactiveconnected radio communication mode.Paragraph 13. The method of paragraph 9, in which placing the terminaldevice into a state comprises performing an intracell handover of theterminal device.Paragraph 14. The method of paragraph 1 or paragraph 2, in which themaster base station effects the alteration in said data handlingresources by resetting its medium access control layer as part of aprocedure in which one of the master base station or the secondary basestation changes the type of radio bearer it handles away from the saidsplit radio bearer.Paragraph 15. A method for use in a wireless telecommunications network,the mobile telecommunications network comprising: a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and a terminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device; themethod comprising: when the secondary base station exhausts a supply ofunique parameter sets used in security ciphering of received split radiobearers, the unique parameter sets each including a first key, thesecondary base station is provided with a second key to create a newsupply of unique parameter sets, the second key is communicated to theterminal device, and the terminal device uses both the first key and thesecond key for handling the split radio bearer until it determines thatthe first key is redundant.Paragraph 16. A method for use in a wireless telecommunications network,the mobile telecommunications network comprising: a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and and a terminal device configured to communicate wirelessly with thebase stations including by the use of a split radio bearer receivable atthe secondary base station for splitting between the secondary basestation and the master base station before delivery to the terminaldevice; the method comprising: when the secondary base station exhaustsa supply of unique parameter sets used in security ciphering of receivedsplit radio bearers, a new parameter is introduced and added to eachparameter set to create a new supply of unique parameter sets.Paragraph 17. A wireless telecommunications network comprising: a corenetwork; base stations supported by the core network and each providingwireless connectivity within at least one base station cell where thecells are arranged into a master cell group under control of a masterbase station and a secondary cell group under control of a secondarybase station; and a terminal device configured to communicate wirelesslywith the base stations including by the use of a split radio bearerreceivable at the secondary base station for splitting between thesecondary base station and the master base station before delivery tothe terminal device; wherein the wireless telecommunications network isconfigured to carry out the method of any one of paragraphs 1 to 16.Paragraph 18. A method of operating a base station in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station; and aterminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device;where the base station is a secondary base station, the methodcomprising: when the secondary base station exhausts a supply of uniqueparameter sets used in security ciphering of received split radiobearers, the secondary base station sends a notification to the masterbase station of a requirement to alter data handling resources allocatedfor handling split radio bearers received from the secondary basestation in one or both of the master base station and the terminaldevice.Paragraph 19. A base station for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station; and a terminal deviceconfigured to communicate wirelessly with the base stations including bythe use of a split radio bearer receivable at the secondary base stationfor splitting between the secondary base station and the master basestation before delivery to the terminal device; wherein the base stationis a secondary base station, and comprises a controller unit and atransceiver unit and is configured to provide wireless connectivitywithin the cells of the secondary cell group, and is configured to: senda notification to the master base station of a requirement to alter datahandling resources allocated for handling split radio bearers receivedfrom the secondary base station in one or both of the master basestation and the terminal device, when the secondary base stationexhausts a supply of unique parameter sets used in security ciphering ofreceived split radio bearers.Paragraph 20. Integrated circuitry for a base station for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and a terminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device;wherein the base station is a secondary base station, and the integratedcircuitry comprises a controller element and a transceiver element andis configured to: enable the secondary base station to provide wirelessconnectivity within the cells of the secondary cell group; and send anotification to the master base station of a requirement to alter datahandling resources allocated for handling split radio bearers receivedfrom the secondary base station in one or both of the master basestation and the terminal device, when the secondary base stationexhausts a supply of unique parameter sets used in security ciphering ofreceived split radio bearers.Paragraph 21. A method of operating a base station in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station; and aterminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device;where the base station is a master base station, the method comprising:receiving at the master base station a notification sent by secondarybase station following exhaustion of a supply of unique parameter setsused by the secondary base station in security ciphering of receivedsplit radio bearers, the notification requiring the master base stationto alter data handling resources allocated for handling split radiobearers received from the secondary base station in one or both of themaster base station and the terminal device; and in response to thenotification, the master base station operating to effect an alterationin the said data handling resources.Paragraph 22. A base station for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station; and a terminal deviceconfigured to communicate wirelessly with the base stations including bythe use of a split radio bearer receivable at the secondary base stationfor splitting between the secondary base station and the master basestation before delivery to the terminal device; wherein the base stationis a master base station, and comprises a controller unit and atransceiver unit and is configured to provide wireless connectivitywithin the cells of the master cell group, and is configured to: receivea notification sent by the secondary base station following exhaustionof a supply of unique parameter sets used by the secondary base stationin security ciphering of received split radio bearers, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice; and in response to the notification, effect an alteration in thesaid data handling resources.Paragraph 23. Integrated circuitry for a base station for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and a terminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device;wherein the base station is a master base station, and the integratedcircuitry comprises a controller element and a transceiver element andis configured to: enable the master base station to provide wirelessconnectivity within the cells of the master cell group; and receive anotification sent by the secondary base station following exhaustion ofa supply of unique parameter sets used by the secondary base station insecurity ciphering of received split radio bearers, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice; and in response to the notification, effect an alteration in thesaid data handling resources.Paragraph 24. A method of operating a terminal device in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station; and theterminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device; themethod comprising: effecting an alteration in data handling resources ofthe terminal device which are allocated for handling split ratio radiobearers received from the secondary base station in response to areconfiguration message sent by the master base station to the terminaldevice in response to receipt by the master base station of anotification sent by the secondary base station following exhaustion ofa supply of unique parameter sets used by the secondary base station insecurity ciphering of received split radio bearers, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice.Paragraph 25. A terminal device for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station; and the terminal device:wherein the terminal device comprises a controller unit and atransceiver unit and is configured to: communicate wirelessly with thebase stations including by the use of a split radio bearer receivable atthe secondary base station for splitting between the secondary basestation and the master base station before delivery to the terminaldevice; and effect an alteration in data handling resources of theterminal device which are allocated for handling split ratio radiobearers received from the secondary base station in response to areconfiguration message sent by the master base station to the terminaldevice in response to receipt by the master base station of anotification sent by the secondary base station following exhaustion ofa supply of unique parameter sets used by the secondary base station insecurity ciphering of received split radio bearers, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice.Paragraph 26. Integrated circuitry for a terminal device for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station;and the terminal device: wherein the integrated circuitry comprises acontroller element and a transceiver element and is configured to:enable the terminal device to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device; andeffect an alteration in data handling resources of the terminal devicewhich are allocated for handling split ratio radio bearers received fromthe secondary base station in response to a reconfiguration message sentby the master base station to the terminal device in response to receiptby the master base station of a notification sent by the secondary basestation following exhaustion of a supply of unique parameter sets usedby the secondary base station in security ciphering of received splitradio bearers, the notification requiring the master base station toalter data handling resources allocated for handling split radio bearersreceived from the secondary base station in one or both of the masterbase station and the terminal device.Paragraph 27. A method for use in a wireless telecommunications network,the mobile telecommunications network comprising: a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station,wherein in the secondary cell group one cell is designated as a primarysecondary cell with responsibility for uplink control signalling in thesecondary cell group; and a terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group via the primary secondarycell for splitting between the secondary base station and the masterbase station before delivery to the terminal device; the methodcomprising: in the event of a need to change the designation of theprimary secondary cell to a different cell in the secondary cell group,the secondary base station sends a notification to the master basestation of a requirement to alter data handling resources allocated forhandling split radio bearers received from the secondary base station inone or both of the master base station and the terminal device; and themaster base station, in response to the notification, operates to effectan alteration in the said data handling resources.Paragraph 28. The method of paragraph 27, in which the master cell groupand the master base station are configured within a Long Term Evolutionradio access network, and the secondary base station is a core unitassociated with distributed units providing the cells of the secondarycell group, the core unit and the distributed units configured under adifferent radio access network, the master cell group and the secondarycell group interworking to provide the wireless telecommunicationsnetwork.Paragraph 29. The method of paragraph 27 or paragraph 28, in which: themaster base station effects the alteration in said data handlingresources by re-establishing its radio link control protocol layer andresetting its medium access control protocol layer, and sending areconfiguration message to the terminal device to effectre-establishment of its radio link control protocol layer and resettingof its medium access control protocol layer.Paragraph 30. The method of paragraph 27 or paragraph 28, in which: themaster base station effects the alteration in said data handlingresources by re-establishing its radio link control protocol layer andnot resetting its medium access control protocol layer, sending areconfiguration message to the terminal device to effectre-establishment of its radio link protocol layer but not resetting ofits medium access control layer.Paragraph 31. The method of paragraph 30, further comprising: arrangingthat a hybrid automatic repeat request buffer in the medium accesscontrol protocol of the said data handling resources in the master basestation is empty before re-establishing its radio link control layer.Paragraph 32. The method of paragraph 31, in which arranging for thebuffer to be empty includes stopping downlink data traffic to the masterbase station for a predetermined time period following exhaustion of thesupply of unique parameter sets.Paragraph 33. The method of paragraph 31, in which arranging for thebuffer to be empty includes stopping uplink data traffic to the masterbase station for a predetermined time period following exhaustion of thesupply of unique parameter sets.Paragraph 34. The method of any one of paragraphs 31 to 33, furthercomprising sending a status report to the secondary base station toindicate an empty status of the buffer, before the secondary basestation sends the notification to the master base station.Paragraph 35. The method of paragraph 27 or paragraph 28, in which themaster base station effects the alteration in said data handlingresources by placing the terminal device into a state in which its saiddata handling resources can no longer handle the split radio bearers.Paragraph 36. The method of paragraph 35, in which placing the terminaldevice into a state comprises moving the terminal device to an idleradio communication mode.Paragraph 37. The method of paragraph 35,in which placing the terminaldevice into a state comprises releasing the terminal device fromconnection with the split radio bearers.Paragraph 38. The method of paragraph 35, in which placing the terminaldevice into a state comprises moving the terminal device to an inactiveconnected radio communication mode.Paragraph 39. The method of paragraph 35, in which placing the terminaldevice into a state comprises performing an intracell handover of theterminal device.Paragraph 40. The method of paragraph 27 or paragraph 28, in which themaster base station effects the alteration in said data handlingresources by resetting its medium access control layer as part of aprocedure in which one of the master base station or the secondary basestation changes the type of radio bearer it handles away from the saidsplit radio bearer.Paragraph 41. A method for use in a wireless telecommunications network,the mobile telecommunications network comprising: a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station,wherein in the secondary cell group one cell is designated as a primarysecondary cell, and the master cell group and the master base stationare configured within a Long Term Evolution radio access network, andthe secondary base station is a core unit associated with distributedunits providing the cells of the secondary cell group, the core unit andthe distributed units configured under a different radio access network,the master cell group and the secondary cell group interworking toprovide the wireless telecommunications network; and a terminal deviceconfigured to communicate wirelessly with the base stations including bythe use of a split radio bearer receivable at the secondary cell groupvia the primary secondary cell for splitting between the secondary basestation and the master base station before delivery to the terminaldevice; the method comprising: in the event of a need to change thedesignation of the primary secondary cell from a first cell to a secondcell in the secondary cell group when the first cell and the second cellare both provided by a same distributed unit so that the first cell andthe second cell share a common medium access control protocol layer, thechange of the primary secondary cell from the first cell to the secondcell is performed using signalling in the common medium access controlprotocol layer.Paragraph 42. A wireless telecommunications network comprising: a corenetwork; base stations supported by the core network and each providingwireless connectivity within at least one base station cell where thecells are arranged into a master cell group under control of a masterbase station and a secondary cell group under control of a secondarybase station, wherein in the secondary cell group one cell is designatedas a primary secondary cell; and a terminal device configured tocommunicate wirelessly with the base stations including by the use of asplit radio bearer receivable at the secondary cell group via theprimary secondary cell for splitting between the secondary base stationand the master base station before delivery to the terminal device;wherein the wireless telecommunications network is configured to carryout the method of any one of paragraphs 27 to 41.Paragraph 43. A method of operating a base station in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station, whereinin the secondary cell group one cell is designated as a primarysecondary cell; and a terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group via the primary secondarycell for splitting between the secondary base station and the masterbase station before delivery to the terminal device; where the basestation is a secondary base station, the method comprising: when a needarises to change the designation of the primary secondary cell to adifferent cell in the secondary cell group, the secondary base stationsends a notification to the master base station of a requirement toalter data handling resources allocated for handling split radio bearersreceived from the secondary base station in one or both of the masterbase station and the terminal device.Paragraph 44. A base station for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station, wherein in the secondary cellgroup one cell is designated as a primary secondary cell; and a terminaldevice configured to communicate wirelessly with the base stationsincluding by the use of a split radio bearer receivable at the secondarycell group for splitting between the secondary base station and themaster base station before delivery to the terminal device; wherein thebase station is a secondary base station, and comprises a controllerunit and a transceiver unit and is configured to provide wirelessconnectivity within the cells of the secondary cell group, and isconfigured to: send a notification to the master base station of arequirement to alter data handling resources allocated for handlingsplit radio bearers received from the secondary base station in one orboth of the master base station and the terminal device, when a needarises to change the designation of the primary secondary cell to adifferent cell in the secondary cell group.Paragraph 45. Integrated circuitry for a base station for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station,wherein in the secondary cell group one cell is designated as a primarysecondary cell; and a terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; wherein the base station is a secondary basestation, and the integrated circuitry comprises a controller element anda transceiver element and is configured to: enable the secondary basestation to provide wireless connectivity within the cells of thesecondary cell group; and send a notification to the master base stationof a requirement to alter data handling resources allocated for handlingsplit radio bearers received from the secondary base station in one orboth of the master base station and the terminal device, when a needarises to change the designation of the primary secondary cell to adifferent cell in the secondary cell group.Paragraph 46. A method of operating a base station in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station, whereinin the secondary cell group one cell is designated as a primarysecondary cell; and a terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; where the base station is a master base station,the method comprising: receiving at the master base station anotification sent by secondary base station when a need arises to changethe designation of the primary secondary cell to a different cell in thesecondary cell group, the notification requiring the master base stationto alter data handling resources allocated for handling split radiobearers received from the secondary base station in one or both of themaster base station and the terminal device; and in response to thenotification, the master base station operating to effect an alterationin the said data handling resources.Paragraph 47. A base station for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station, wherein in the secondary cellgroup one cell is designated as a primary secondary cell; and a terminaldevice configured to communicate wirelessly with the base stationsincluding by the use of a split radio bearer receivable at the secondarycell group for splitting between the secondary base station and themaster base station before delivery to the terminal device; wherein thebase station is a master base station, and comprises a controller unitand a transceiver unit and is configured to provide wirelessconnectivity within the cells of the master cell group, and isconfigured to: receive a notification sent by the secondary base stationwhen a need arises to change the designation of the primary secondarycell to a different cell in the secondary cell group, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice; and in response to the notification, effect an alteration in thesaid data handling resources.Paragraph 48. Integrated circuitry for a base station for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base station,wherein in the secondary cell group one cell is designated as a primarysecondary cell; and a terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; wherein the base station is a master base station,and the integrated circuitry comprises a controller element and atransceiver element and is configured to: enable the master base stationto provide wireless connectivity within the cells of the master cellgroup; and receive a notification sent by the secondary base stationwhen a need arises to change the designation of the primary secondarycell to a different cell in the secondary cell group, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice; and in response to the notification, effect an alteration in thesaid data handling resources.Paragraph 49. A method of operating a terminal device in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station, whereinin the secondary cell group one cell is designated as a primarysecondary cell; and the terminal device configured to communicatewirelessly with the base stations including by the use of a split radiobearer receivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; the method comprising: effecting an alteration indata handling resources of the terminal device which are allocated forhandling split ratio radio bearers received from the secondary basestation in response to a reconfiguration message sent by the master basestation to the terminal device in response to receipt by the master basestation of a notification sent by the secondary base station when a needarises to change the designation of the primary secondary cell to adifferent cell in the secondary cell group, the notification requiringthe master base station to alter data handling resources allocated forhandling split radio bearers received from the secondary base station inone or both of the master base station and the terminal device.Paragraph 50. A terminal device for use in a wireless telecommunicationsnetwork which comprises a core network; base stations supported by thecore network and each providing wireless connectivity within at leastone base station cell where the cells are arranged into a master cellgroup under control of a master base station and a secondary cell groupunder control of a secondary base station wherein in the secondary cellgroup one cell is designated as a primary secondary cell; and theterminal device: wherein the terminal device comprises a controller unitand a transceiver unit and is configured to: communicate wirelessly withthe base stations including by the use of a split radio bearerreceivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; and effect an alteration in data handling resourcesof the terminal device which are allocated for handling split ratioradio bearers received from the secondary base station in response to areconfiguration message sent by the master base station to the terminaldevice in response to receipt by the master base station of anotification sent by the secondary base station when a need arises tochange the designation of the primary secondary cell to a different cellin the secondary cell group, the notification requiring the master basestation to alter data handling resources allocated for handling splitradio bearers received from the secondary base station in one or both ofthe master base station and the terminal device.Paragraph 51. Integrated circuitry for a terminal device for use in awireless telecommunications network which comprises a core network; basestations supported by the core network and each providing wirelessconnectivity within at least one base station cell where the cells arearranged into a master cell group under control of a master base stationand a secondary cell group under control of a secondary base stationwherein in the secondary cell group one cell is designated as a primarysecondary cell; and the terminal device: wherein the integratedcircuitry comprises a controller element and a transceiver element andis configured to: enable the terminal device to communicate wirelesslywith the base stations including by the use of a split radio bearerreceivable at the secondary cell group for splitting between thesecondary base station and the master base station before delivery tothe terminal device; and effect an alteration in data handling resourcesof the terminal device which are allocated for handling split ratioradio bearers received from the secondary base station in response to areconfiguration message sent by the master base station to the terminaldevice in response to receipt by the master base station of anotification sent by the secondary base station when a need arises tochange the designation of the primary secondary cell to a different cellin the secondary cell group, the notification requiring the master basestation to alter data handling resources allocated for handling splitradio bearers received from the secondary base station in one or both ofthe master base station and the terminal device.

REFERENCES

[1] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based radioaccess”, John Wiley and Sons, 2009

1. A method for use in a wireless telecommunications network, the mobiletelecommunications network comprising: a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station; and aterminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary base station for splitting between the secondary base stationand the master base station before delivery to the terminal device; themethod comprising: when the secondary base station exhausts a supply ofunique parameter sets used in security ciphering of received split radiobearers, the secondary base station sends a notification to the masterbase station of a requirement to alter data handling resources allocatedfor handling split radio bearers received from the secondary basestation in one or both of the master base station and the terminaldevice; and the master base station, in response to the notification,operates to effect an alteration in the said data handling resources. 2.The method of claim 1, in which the parameter sets are made unique byinclusion of successive numbers from a counter in a packet dataconvergence protocol that receives, ciphers and splits the split radiobearer, and exhaustion of the supply of unique parameter sets occurswhen the counter rolls over from its final number back to its startingnumber.
 3. The method of claim 1, in which: the master base stationeffects the alteration in said data handling resources byre-establishing its radio link control protocol layer and resetting itsmedium access control protocol layer, and sending a reconfigurationmessage to the terminal device to effect re-establishment of its radiolink control protocol layer and resetting of its medium access controlprotocol layer.
 4. The method of claim 1, in which; the master basestation effects the alteration in said data handling resources byre-establishing its radio link control protocol layer and not resettingits medium access control protocol layer, sending a reconfigurationmessage to the terminal device to effect re-establishment of its radiolink protocol layer but not resetting of its medium access controllayer.
 5. The method of claim 4, further comprising: arranging that ahybrid automatic repeat request buffer in the medium access controlprotocol of the said data handling resources in the master base stationis empty before re-establishing its radio link control layer.
 6. Themethod of claim 5, in which arranging for the buffer to be emptyincludes stopping downlink data traffic to the master base station for apredetermined time period following exhaustion of the supply of uniqueparameter sets.
 7. The method of claim 5, in which arranging for thebuffer to be empty includes stopping uplink data traffic to the masterbase station for a predetermined time period following exhaustion of thesupply of unique parameter sets.
 8. The method of claim 5, furthercomprising sending a status report to the secondary base station toindicate an empty status of the buffer, before the secondary basestation sends the notification to the master base station.
 9. The methodof claim 1, in which the master base station effects the alteration insaid data handling resources by placing the terminal device into a statein which its said data handling resources can no longer handle the splitradio bearers.
 10. The method of claim 9, in which placing the terminaldevice into a state comprises moving the terminal device to an idleradio communication mode.
 11. The method of claim 9, in which placingthe terminal device into a state comprises releasing the terminal devicefrom connection with the split radio bearers.
 12. The method of claim 9,in which placing the terminal device into a state comprises moving theterminal device to an inactive connected radio communication mode. 13.The method of claim 9, in which placing the terminal device into a statecomprises performing an intracell handover of the terminal device. 14.The method of claim 1, in which the master base station effects thealteration in said data handling resources by resetting its mediumaccess control layer as part of a procedure in which one of the masterbase station or the secondary base station changes the type of radiobearer it handles away from the said split radio bearer. 15-24.(canceled)
 25. A terminal device for use in a wirelesstelecommunications network which comprises a core network; base stationssupported by the core network and each providing wireless connectivitywithin at least one base station cell where the cells are arranged intoa master cell group under control of a master base station and asecondary cell group under control of a secondary base station; and theterminal device: wherein the terminal device comprises a controller unitand a transceiver unit and is configured to: communicate wirelessly withthe base stations including by the use of a split radio bearerreceivable at the secondary base station for splitting between thesecondary base station and the master base station before delivery tothe terminal device; and effect an alteration in data handling resourcesof the terminal device which are allocated for handling split ratioradio bearers received from the secondary base station in response to areconfiguration message sent by the master base station to the terminaldevice in response to receipt by the master base station of anotification sent by the secondary base station following exhaustion ofa supply of unique parameter sets used by the secondary base station insecurity ciphering of received split radio bearers, the notificationrequiring the master base station to alter data handling resourcesallocated for handling split radio bearers received from the secondarybase station in one or both of the master base station and the terminaldevice.
 26. (canceled)
 27. A method for use in a wirelesstelecommunications network, the mobile telecommunications networkcomprising: a core network; base stations supported by the core networkand each providing wireless connectivity within at least one basestation cell where the cells are arranged into a master cell group undercontrol of a master base station and a secondary cell group undercontrol of a secondary base station, wherein in the secondary cell groupone cell is designated as a primary secondary cell with responsibilityfor uplink control signalling in the secondary cell group; and aterminal device configured to communicate wirelessly with the basestations including by the use of a split radio bearer receivable at thesecondary cell group via the primary secondary cell for splittingbetween the secondary base station and the master base station beforedelivery to the terminal device; the method comprising: in the event ofa need to change the designation of the primary secondary cell to adifferent cell in the secondary cell group, the secondary base stationsends a notification to the master base station of a requirement toalter data handling resources allocated for handling split radio bearersreceived from the secondary base station in one or both of the masterbase station and the terminal device; and the master base station, inresponse to the notification, operates to effect an alteration in thesaid data handling resources.
 28. The method of claim 27, in which themaster cell group and the master base station are configured within aLong Term Evolution radio access network, and the secondary base stationis a core unit associated with distributed units providing the cells ofthe secondary cell group, the core unit and the distributed unitsconfigured under a different radio access network, the master cell groupand the secondary cell group interworking to provide the wirelesstelecommunications network. 29-34. (canceled)
 35. The method of claim27, in which the master base station effects the alteration in said datahandling resources by placing the terminal device into a state in whichits said data handling resources can no longer handle the split radiobearers.
 36. The method of claim 35, in which placing the terminaldevice into a state comprises moving the terminal device to an idleradio communication mode. 37-49. (canceled)
 50. A terminal device foruse in a wireless telecommunications network which comprises a corenetwork; base stations supported by the core network and each providingwireless connectivity within at least one base station cell where thecells are arranged into a master cell group under control of a masterbase station and a secondary cell group under control of a secondarybase station wherein in the secondary cell group one cell is designatedas a primary secondary cell; and the terminal device: wherein theterminal device comprises a controller unit and a transceiver unit andis configured to: communicate wirelessly with the base stationsincluding by the use of a split radio bearer receivable at the secondarycell group for splitting between the secondary base station and themaster base station before delivery to the terminal device; and effectan alteration in data handling resources of the terminal device whichare allocated for handling split ratio radio bearers received from thesecondary base station in response to a reconfiguration message sent bythe master base station to the terminal device in response to receipt bythe master base station of a notification sent by the secondary basestation when a need arises to change the designation of the primarysecondary cell to a different cell in the secondary cell group, thenotification requiring the master base station to alter data handlingresources allocated for handling split radio bearers received from thesecondary base station in one or both of the master base station and theterminal device.
 51. (canceled)